1. Field of the Invention
The present invention relates to an apparatus for automatically and selectively discharging saline water at a tide-preventing floodgate between the ocean and a river, a lake, a canal or the like, more particularly, to a field-type apparatus for automatically and selectively discharging saline water at a tide-preventing floodgate between the ocean and a river or the like, without using any prime mover and in the absence of a controller.
2. Description of the Prior Art
Tide-preventing floodgates are usually provided at places where the ocean contacts with rivers, waterways, canals, lakes or the like to prevent invasion of sea water of the downstream side of the floodgate to river water or the like, thereby to utilize upstream fresh water as water resources. Tide-preventing floodgates have also a function to discharge incoming water from upstream to downstream side of the floodgate, if the incoming water is accumulated too much and incurring a danger of flood damage.
When a water gate is pulled up to discharge accumulated upstream water to downstream or when a gate of accompanied establishment of a tide-preventing floodgate is opened to pass a ship, some sea water flows upstream or invades upstream through the gate. On the upstream side of the gate, there is also saline water coming from the sea water of the downstream through the bottom soil by penetration. Therefore, usually saline water having a high density appears at a bottom layer portion of a river or the like, and fresh water having a low density appears at upper layer portion of a river or the like.
It is extremely difficult to exclusively and selectively discharge saline water having a high density in a bottom layer portion of a river or the like at a floodgate, because there is an increase or a decrease of incoming water at upstream side of the floodgate and there is also a change of sea water level due to tide at downstream side of the floodgate, so that water level or levels at the floodgate always changes. Pulling up of a tide-preventing floodgate is usually effected in such a situation that incoming water has been accumulated at upstream side of a floodgate and water level of a river is raised too much and there is a risk of damage due to flood of the river. Even in such a situation, when the gate is pulled up, sea water flows upstream, though comparatively small in quantity, from bottom portion of the opening of the gate, unless upstream water level is higher than downstream sea water level by a value .DELTA.Hc, as shown in FIG. 3.
A conventional tide-preventing floodgate is shown in FIGS. 1-3, wherein a floodgate 1 closes a waterway 2. Sea water 3 is on the right of the floodgate 1 and fresh water 4 is on the left of the floodgate 1. A density of sea water is .rho..sub.2 and its depth is shown by h.sub.1. A level L.sub.2 of the sea water receives an influence of the tide and changes twice a day as shown by a curve L. Changes of sea water level L.sub.2 varies depending on regions, day by day and season by season and fluctuates considerably. Incoming water flows from upstream to downstream of a river, a fresh water lake, a waterway or the like, and if water flowed into is left as it is, the water level L.sub.1 of fresh water rises and there is flood damage, therefore, it is necessary to pull up the gate for draining. In this case, sea water flows upstream through the gate, if the level L.sub.1 on the upstream side of the floodgate is not higher than the level L.sub.2 of sea water by a level difference limit .DELTA.Hc that prevents sea water from flowing upstream through the gate. The level difference limit .DELTA.Hc is about 3% of the sea water depth h.sub.1. If the gate is pulled up when the water level difference .DELTA.H between the fresh water level L.sub.1 and the sea water level L.sub.2 is less than the level difference limit .DELTA.Hc, a situation occurs that a surface layer fresh water H.sub.1 flows out through an upper part of the opening of the gate, while sea water 3a flows through a lower part of the opening of the gate into lower portion of upstream fresh water, thus forming a saline water layer H.sub.2 having a density .rho.'.sub.2 as shown in FIGS. 3 and 4.
It is therefore necessary to operate a floodgate 1 by a supervisor with careful attention. However, where a ship fairway is arranged besides a tide-preventing floodgate, sea water flows upstream regardless of a supervisor's control when the ship fairway is opened. Therefore, heretofore, an ideal control of a floodgate has been very difficult, incomplete or very complicated. In addition, tide-preventing floodgates have often been constructed at remote or inconvenient places where no prime mover is applicable. Hence, a developing of an apparatus for automatically and selectively discharge saline water solely, without using a prime mover and without needing an operator, has been earnestly desired.
An object of the invention is, therefore, to provide an apparatus for automatically and selectively discharging saline water at a tide-preventing floodgate to preserve fresh water of a river or the like as much as possible to permit maximum effective utilization of fresh water resources.
Another object of the invention is to provide an apparatus for automatically and selectively discharging saline water at a tide-preventing floodgate at places where transportation is inconvenient or remote places.
Another object of the invention is to provide an apparatus for automatically and selectively discharging saline water without using a prime mover and without needing an operator.
Another object of the invention is to provide a cheap and simple apparatus for automatically and selectively discharging saline water.
A further object of the invention is to provide an apparatus for automatically and selectively discharging saline water at a tide-preventing floodgate to prevent a salt damage of upstream side areas of a river or the like due to saline water.
These and other objects of the invention will be made apparent from the following more particular description of the invention.